1. Field of the Invention
This invention relates generally to a magnetic disc reproducing apparatus and, in particular, is directed to a magnetic disc reproducing apparatus in which a flexible disc cartridge (floppy disk) for an electronic still camera is used alternatively as a medium for storing digital data.
2. Description of the Prior Art
A prior art 8-inch or 5.25-inch floppy disk is standardized in its format and almost all of the disk drives are operated in accordance with that standardized format. As a result, it is very difficult to employ advanced technology for realizing higher density recording and/or reproducing in the prior art system. Further, since the rotation speed of a magnetic disk incorporated in the prior art floppy disk system is usually 300 r.p.m. or 600 r.p.m., it is impossible to use such system to record and/or reproduce an analog video signal in real time. If the video signal is digitized, it can be recorded and/or reproduced, but then one floppy disk is capable of at best recording only one still video picture if its receptacle jacket size is to be maintained at 8-inch or 5.25-inch. Furthermore, for such use of the known system in addition to an A/D (analog-to-digital) converter and a D/A (digital-to-analog) converter, the frame memory is required so that a total system becomes very expensive and large in size.
Accordingly, it is not practical to use the prior art floppy disk system to record and/or reproduce the video signal.
An electronic still camera conference convened in Japan therefore proposed a 2-inch floppy disk as a recording medium for an electronic still camera. FIG. 1 is a diagram of such 2-inch floppy disk and illustrates the general construction thereof.
In FIG. 1, reference numeral 1 generally designates the proposed floppy disk and reference numeral 2 designates a flexible magnetic disk incorporated in the floppy disk 1. The magnetic disk 2 is 47 mm in diameter and 40 .mu.m in thickness and is provided at its center portion with a center core 3 with which the spindle of a drive mechanism (not shown) is engageable. The center core 3 is provided with a magnetic piece or member 4 which can be used for sensing the rotational position when the magnetic disk 2 rotates.
Reference numeral 5 designates a receptacle or jacket for the magnetic disk 2. This jacket 5 is 60.times.54.times.3.6 mm in size and has the magnetic disk 2 freely rotatable therein. The jacket 5 includes a central opening 5A to expose therethrough the center core 3 and the magnetic piece 4 to the outside. The jacket 5 is further provided with another opening 5B through which a magnetic head (not shown) can contact with the magnetic disk 2 upon recording and/or reproducing. When the floppy disk 1 is not in use, the opening 5B is closed by a slidable dust-proof shutter 6. Reference numeral 7 designates a counter dial for indicating the number of pictures taken by an electronic camera and reference numeral 8 designates a tab member the presence or absence of which can be detected for preventing the accidental recording of signals on disk 2 when information already recorded thereon is to be preserved. The tab member 8 is removed when recording is to be inhibited.
Upon recording, 50 magnetic tracks can be concentrically formed on one surface of the magnetic disk 2 and the outermost track is represented as the first track and the innermost track is represented as the 50th track. Each track width is 60 .mu.m and the guard band width between tracks is 40 .mu.m, respectively.
In taking a picture, the magnetic disk 2 is rotated at 3600 r.p.m. (field frequency) and a color video signal of one field is recorded in a selected single track of the magnetic disk 2. In this case, the color video signal Sa to be recorded includes, as shown in FIG. 2, a luminance signal Sy which is frequency-modulated to a frequency modulated signal Sf, wherein the sync tip level of this signal is 6 MHz and the white peak level is 7.5 MHz. For a chrominance signal in the signal Sa to be recorded, there is formed a line sequential color signal Sc which consists of a frequency modulated red color difference signal (center frequency is 1.2 MHz) and a frequency modulated blue color difference signal (center frequency is 1.3 MHz). The signal Sa is obtained by adding the frequency modulated color signal Sc and the frequency modulated luminance signal Sy for recording on the magnetic disk 2.
As described above, the floppy disk 1 shown in FIG. 1 has the proper size, function and characteristics for use as a recording medium for 50 still color video signals.
Since, however, the floppy disk 1 was first standardized to record and/or reproduce an analog color video signal as set forth above, it can not easily handle digital data. If, for example, digital data is converted to quasi video signal and then recorded on the floppy disk 1 just as in an audio PCM (pulse code modulation) processor for a VTR (video tape recorder), the floppy disk 1 is small in memory capacity for the original digital data and also there are many other problems, such as, a lack of data compatibility with an existing 8-inch or 5.25-inch floppy disk, a difference of broadcasting system, an undesirably enlarged circuit, etc.
Alternatively, when the color video signal is recorded on or reproduced from the floppy disk 1, such operations are carried out in accordance with the above-mentioned format, while when the digital data is recorded on or reproduced therefrom, it is carried out in accordance with the format of the prior art floppy disk. In that case, when the floppy disk 1 is viewed as a recording medium for the video signal, it has a very high recording density 7, while when it is used for recording the digital data, it has a low recording density 7 and thus, full use of the floppy disk 1 is not being achieved.
When the video signal and the digital data are recorded on or reproduced from one floppy disk 1 in an intermixed manner, both signals have largely different frequency bands and characteristics so that it becomes difficult to record and reproduce the video signal and the digital data together under optimum conditions in respect to the electromagnetic transducer characteristic, head-disk contact condition and so on. Further, when the video signal and the digital data are recorded and/or reproduced in an intermixed manner on one floppy disk 1, the drive unit for rotating the floppy disk 1 must be rotated at 300 r.p.m. (600 r.p.m.) when recording or reproducing digital data and at 3600 r.p.m. for the video signal, so that when the revolution speed of the floppy disk 1 is selectively changed, problems occur, such as, the floppy disk 1 cannot be accessed for several seconds until the servo is stabilized, the manufacturing cost becomes increased and so on.
Therefore, it has been considered that the floppy disk 1 should use the following format so as to become able to appropriately record and reproduce the video signal and also the digital data.
In FIG. 3A, reference numeral 2T designates one of the tracks on the magnetic disk 2. This track 2T is equally divided into four 90.degree. intervals in its circumferential direction with the magnetic piece 4 as a reference. Each of the four divided intervals is called a block BLCK and the block BLCK of the interval containing the magnetic piece 4 is identified as block 0 and the succeeding three blocks are represented as block 1, block 2 and block 3, sequentially.
As shown in FIG. 3B, in each block BLCK, an interval of 4.degree. from its beginning is represented as a gap interval GAP which affords a margin upon reading and writing. A succeeding interval of 1.degree. is represented as a burst interval BRST. In this case, in the block 0 BLCK, the center of the gap interval GAP corresponds to the position of the magnetic piece 4. The burst interval BRST is an interval in which there is recorded and/or reproduced a burst signal BRST which serves as
(i) a preamble signal PA1 (ii) a signal indicative of a recording density of a recorded signal; and PA1 (iii) a flag signal indicating that the respective recorded signal is a digital signal. PA1 One block: 4096 bytes (=32 bytes.times.128 frames) PA1 one track: 16K bytes (=4096 bytes.times.4 blocks) PA1 one disk: 800K bytes (=16K bytes.times.50 tracks) PA1 One frame: 352 bits=(8+16+8 bits)+(16+4 bytes).times.8 bits.times.2 frames) PA1 one block (only index interval and frame interval): 45120 bits (=352 bits.times.123 frames) In practice, however, when the digital signal is recorded on or reproduced from the disk 2, a DSV (digital sum value) is required to be small and a ration Tmin (minimum length between transition)/Tmax (maximum length between transition) is required to be small, while a window margin Tw is required to be large. Thus, all of the afore-described digital signals are subjected to 8/10 (eight-to-ten)-conversion with Tmax=4T and then recorded on the disk 2. Upon reproducing, the digital signals are subjected to a reverse conversion and then subjected to the succeeding inherent signal processing. PA1 one frame: 440 channel bits PA1 one block (only the index interval and frame interval): 56400 channel bits Thus, the total number of the bits in the whole interval of one block corresponds to 59719 channel bits (.congruent.56400 channel bits.times.90.degree./85.degree.). Since, in practice, the length of each interval is assigned by the number of the channel bits as mentioned above, the total angle of the frame intervals is a little shorter than 85.degree.. PA1 14.32 M bits/sec (.congruent.59719 bits.times.4 blocks.times.field frequency) and one bit corresponds to 69.8 nano seconds (.congruent.1/14.32 M bits). PA1 transducer means placed in transducing relation with said plurality of tracks for reading out said information signals from said tracks; PA1 means connected to said transducer means for checking a portion of said information signals; and PA1 means connected to said checking means for identifying characteristics of said information signals being reproduced and thereby discriminating between different types of the information signals.
The burst interval BRST is followed by an interval for an index signal INDX. In this case, as shown in FIG. 3C, the index signal INDX consists of a flag signal FLAG of 8 bits, an address signal IADR of 8 bits, a reserved signal RSVD of 40 bits and a check signal ICRC of 8 bits. The flag signal FLAG is to indicate whether the track 2T to which the block BLCK belongs is defective or not or whether the track 2T is erased or not and so on. The address signal IADR is to indicate the number (1 to 50) of the track 2T and the number (0 to 3) of the block BLCK, and the check signal ICRC is a CRCC (cyclic redundancy check code) for the flag signal FLAG, the address signal IADR and the reserved signal RSVD.
An interval which follows the index interval INDX is equally divided into 128 frame intervals and a signal identified as a frame FRM is recorded on or reproduced from each of these intervals.
More particularly, as shown in FIG. 3D, each frame FRM includes from its beginning sequentially a frame synchronizing signal SYNC of 8 bits, a frame address signal FADR of 16 bits, a check signal FCRC of 8 bits, digital data DATA of 16 bytes (one byte=8 bits), redundant or parity data PRTY of 4 bytes, another digital data DATA of 16 bytes and another redundant or parity data PRTY of 4 bytes. In this case, the check signal FCRC is a CRCC for the frame address signal FADR. The digital data DATA is the original data which should be accessed by a host computer or apparatus and this digital data DATA is interleaved within a period of digital data of one block BLCK. The redundant data PRTY are parity data C.sub.1 and C.sub.2 which are generated by the Reed Solomon coding method having minimum distance 5 for digital data of one block (32 bytes.times.128 frames).
Accordingly, the capacities for digital data of one block BLCK, one track 2T and one disk 1 are as follows:
The numbers of bits in one frame FRM and one block BLCK are as follows:
Accordingly, in the case of the above-described data density, the practical number of the bits in the disk 2 is multiplied by 10/8 and presented as:
Consequently, the bit rate at which the disk 2 is accessed by the digital signal after its 8/10-conversion) is presented as
As described above, according to the format shown in FIGS. 3A-3D, digital data of 800K bytes can be written on or read out from the floppy disk 1 of 2-inch size and this capacity is more than twice the capacity (320K bytes) of the prior art 5.25-inch floppy disk. Thus, this 2-inch floppy disk 1 has a large capacity in spite of its small size.
Since the disk 2 is rotated at the same rotational speed in the case of recording digital data as in the case of a color video signal recording, when the color video signal and the digital data are recorded on or reproduced from the disk 2 in an intermixed manner, both signals to be recorded on or reproduced from the disk 2 become similar in frequency spectrum and so on, so that they can be recorded on or reproduced from the disk 2 under the optimum conditions such as the electromagnetic conversion characteristic, the contact condition with a magnetic head and the like. Further, even when the two signals are recorded on or reproduced from the disk 2 in an intermixed manner, since the rotational speed of the disk 2 is not changed over, it is not necessary to afford extra time to change-over the servo circuit and thus the two signals can be selectively used immediately. In addition, since the only a single rotational speed of the disk 2 is used and a mechanism such as an electromagnetic transducer system or the like has the same characteristic and function for both types of signals this is advantageous from the standpoint of costs.
Even though the floppy disk 1 of FIG. 1 was originally intended for the analog signal as mentioned above, if the format of FIG. 3 is applied thereto, the floppy disk 1 can achieve a new effect as a floppy disk of the next generation.
By the way, in the case of the prior art floppy disk, the data transfer between it and the peripheral instruments is carried out directly at a speed determined by the rotational speed of the disk without using a buffer memory therebetween. Further, data is allocated on the floppy disk such that data is written in or read out from the floppy disk at sector unit with its address data being made consecutive. In other words, the time sequence of the data recorded on the floppy disk is continuous relative to the original time sequence.
However, in the above-mentioned floppy disk of the next generation, higher transfer speed is required as the digital magnetic recording becomes higher in density. Furthermore, since the data is added with a redundant bit for error correction and re-arranged by interleaving, the time sequence of the resultant data is not the same as the original time sequence on the floppy disk. As a result, the floppy disk can not be directly connected with the peripheral instruments.
Therefore, it may be considered to interpose a buffer memory between the floppy disk and the peripheral instruments. If, however, the data having the data allocation corresponding to the recorded pattern on the floppy disk is written in or read out from the buffer memory as it is, the logical data address as seen from the peripheral instruments is not continuous. Accordingly, when the data is transferred in a DMA (direct memory access) manner so as to transfer the data at high speed, since the data must be transferred with its address being consecutive, such address is not matched with the buffer memory, and thence the data can not be transferred at high speed.
To overcome the above-described shortcoming, it may be considered that upon recording, after the addition of the parity data C.sub.1 and C.sub.2, and upon reproducing, after the error correction process, the data within the buffer memory is re-arranged such that the data is stored in the buffer memory with its address successive in respect to the original time sequence. This, however, requires extra memory capacity and a conversion time for re-arranging the data, which is not desirable.